refactor: le paquet s'appelle polyfill, « SDK » désigne celui de NextGraph

Le nom @ng-eventually/sdk entrait en collision avec le SDK de NextGraph, dont
ce paquet est justement un polyfill. Impossible d'écrire « le SDK » sans lever
l'ambiguïté à chaque phrase — et le contrat publié, lu par une application,
était le pire endroit pour laisser traîner ça.

packages/sdk → packages/polyfill, @ng-eventually/sdk → @ng-eventually/polyfill,
contract_sdk-surface → contract_polyfill-surface, e2e/sdk-entry.ts →
e2e/polyfill-entry.ts, docs/sdk-reference.md → docs/polyfill-reference.md.

Les occurrences de « SDK » qui désignent celui de NextGraph restent intactes,
y compris les chemins dans nextgraph-rs (sdk/js/orm, sdk/js/web). Le tri s'est
fait occurrence par occurrence, pas par substitution.

Le contrat énonce désormais son identité en une phrase : « This package is a
polyfill of NextGraph's SDK. »

docs/read-model.md

@@ -5,7 +5,7 @@ shared wallet. This is a design decision, grounded entirely in the query
 capability documented in
 [`nextgraph-current-state.md`](./nextgraph-current-state.md) § *The query
 capability*. The consumer application never sees any of this: it asks
-`@ng-eventually/sdk` for its lists by need and trusts the answer — the whole
+`@ng-eventually/polyfill` for its lists by need and trusts the answer — the whole
 read mechanism lives here, in the polyfill.
 
 > The rule in one line: read each by-need doc with its own anchored
@@ -111,7 +111,7 @@ Keep the reactive ORM strictly to already-opened single stores; it is a change
 
 ## The boundary with the consumer application
 
-The consumer application asks the SDK for its lists by need and trusts the returned
+The consumer application asks the polyfill for its lists by need and trusts the returned
 set. It never constructs a NURI, never picks the union-vs-anchor mode, never touches
 the ORM. The domain-shaped list helpers (e.g. "my meeting points", "events") live in
 the consumer application, not the lib; the lib exposes the generic by-need read.
@@ -172,7 +172,7 @@ and never iterates the other named graphs. (A repo absent from `self.repos` thro
 `RepoNotFound` and is skipped per-doc, see the VERIFIED note above — the read cannot
 sync an unknown repo.)
 
-> **Re-confirmed by the standing e2e harness (`packages/sdk/e2e/`, broker
+> **Re-confirmed by the standing e2e harness (`packages/polyfill/e2e/`, broker
 > `@ng-org/web 0.1.2-alpha.13`).** The `docRoundTrip` check measures all three shapes
 > anchored to a doc D: (a) a no-GRAPH default-graph write round-trips; (b) an explicit
 > `INSERT DATA { GRAPH <D> {…} }` — a **constant** plain NURI — **also** round-trips